The subjunctive can be quite tricky for English speakers as it is rarely used in the English language. There are several cases in which one should use the subjunctive when speaking Italian:
DOUBT: when there's doubt and expressions like 'Credo (I believe), Penso (I think), Suppongo (I suppose)" are used, the subjunctive must be used. For instance: I believe he's coming = Credo che venga. I don't think she likes it = Non penso che le piaccia.
IMPERSONAL EXPRESSIONS: when impersonal expressions such as "È possible che (It is possible that), È meglio che (It's better that), È necessario che (It's necessary that)", the subjunctive must follow. For instance: It is possible that it will rain = È possible che piova. It's likely that he will show up = È possible che si presenterà.
WISH: when wishes or orders are expressed and expressions such as "Voglio che (I want that), Desidero che (I wish that)" are used, the subjunctive must be used. For instance: Luisa vuole che vada con lei = Luisa wants me to go with her; Vorrei che potessi venire alla mia festa = I wish you could come to my party.
FIXED EXPRESSIONS: when using expressions like "Benchè (Even though), A meno che (Unless), Nonostante (Despite), the subjunctive must follow: A meno che non prenda la patente, non posso guidare = Unless I get my license, I can't drive. Benchè sia malata, andrò comunque in palestra = Even though I'm sick, I will still go to the gym.